1. Field of the Invention
The present invention relates generally to a line pressure control arrangement for an automatic transmission which includes an adaptive self-adjusting feature which compensates for wear and/or temperature changes, and more specifically to such a system which modifies the updating process in accordance with the use of auxiliary devices which consume engine power output and which reduce the amount of torque which is actually supplied to the transmission to drive the vehicle.
2. Description of the Prior Art
JP-A-63-92863 discloses a transmission arrangement of the nature wherein, in order to obviate the deterioration of shift characteristics due to friction element wear and/or changes in hydraulic fluid temperature and the like, the level, to which the line pressure should be controlled, is frequently updated by determining the actual time period defined between the point in time at which a given shift initiates and the point in time at which the shift is completed, with a predetermined optimal time. In the event that the actual period required for the shift to take place is found to be longer than the predetermined one, the level of the line pressure is incremented and vice versa.
However, with this type of arrangement in the event that auxiliary devices such as air conditioners and the like are provided, the amount of engine torque, which is required to drive the compression during use of the same, is substantial and reduces the actual amount of torque which is being supplied to the input shaft of the transmission. This creates a problem in that, during the use of such power consuming devices, the line pressure tends to be adjusted to a level lower than normal in order to compensate for the reduced amount of torque being supplied to the transmission. Accordingly, if the use of the power consuming device is terminated, the amount of engine torque which was used to drive the same is then added to the torque being supplied to the transmission. This brings about the situation wherein the level of line pressure is temporarily set at a level which is inadequate for the increased amount of torque being currently supplied and tends to result in friction element slippage and the like. Conversely, if the power consuming device is switched on after a period of non-use, the reverse situation, wherein the line pressure is set at level which is higher than that required for the amount of torque being supplied to the transmission input shaft, tends to come about. Under these conditions the excessively high line pressure level tends to engage the friction elements more abruptly than desired and leads to the generation of shift and/or select shock.